This invention relates to apparatus and methods for uniformly removing material across the whole width of a substrate by plasma etching to expose the ends of buried, filled vias or other buried features.
The problem of end point detection is well known in the plasma etch industry and various techniques have been developed to detect the point at which a substrate has been etched to the desired depth, so that the etch process can be halted. In typical applications, features are etched through a mask layer into the underlying layer or layers. The detection of the end point (the reaching of the desired etch depth) tends to be based on two distinct technologies. In one, the apparatus or operator is looking for a change in chemistry at the point at which the etch breaks through from one layer to another. This may arise from a change in the chemistry of the actual layers built up in the device or thin etch stop layer may be deliberately deposited between two layers so as to induce a change in chemistry. In early days the etch was simply carried on for a fixed time. More sophisticated and faster techniques have been developed over the years. The second technique is based on reflectometry or interferometry which rely on the upper surface of the substrate (e.g. the hard mask) forming a reference surface against which the depth of the etched structure may be judged.
Recently, 3-D device integration schemes have started to produce new challenges for process equipment. A possible process and equipment flow is shown in FIG. 1, which is Figure EP17 from the assembly and packaging section of the 2009 edition of the ITRS (International Technology Roadmap for Semiconductors). This illustrates the progressive development of the device and, in the highlighted part of the sequence, calls for sequential thinning. In this step a device in which filled vias had been formed is inverted so that the backside of the substrate can be removed down to the level of the tops of the filled vias or other buried feature.
Traditionally, this might all have been done by chemical mechanical polishing but there are advantages in performing the later stages of the step using a plasma etch process. US-B2-7416648 is an example of such a process. However, because the substrate is etched across its width, there is no reference surface by means of which reflectometry or interferometry could be used. Equally, the buried features are so small (typically 10 μm diameter), and form such a small part of the total area of the etch face, that any change in chemistry is too small to detect. The whole top surface etching process is schematically illustrated in FIG. 2. If the thickness of material above the buried features is significant, then it may be convenient to use chemical mechanical polishing until quite close to the expected positions of the tips of the buried features, e.g. within 10 μm and then to etch. FIG. 1(a) is illustrative of a pre-etch situation.